This invention relates to an ultrasonic medical apparatus for applying ultrasonic energy, whether generated inside or outside a patient's body, to a target area for medical treatment within the body by heating or destroying. In particular, the invention relates to an ultrasonic applicator provided with an ultrasonic transducer for such an apparatus.
In medical apparatus of this kind, it has been known to make use of an array of many planar transducers as shown in FIG. 3 so as to provide a suitable ultrasonic wave distribution within the body and to thereby electronically focus these waves at the target position, that is, by using delay circuits to control the phases of high-frequency power from a pulser (or an oscillator) for the individual transducers. Another known method for the same purpose has been to provide an acoustic lens to a planar ultrasonic transducer as shown in FIG. 4.
According to the prior art technology shown in FIG. 4, the planar transducer and the acoustic lens are in close contract with each other, and they are usually circular or quadrangular. For use inside a body cavity, in particular, a thin, elongated rectangular shape is usually adopted, and the acoustic lens is usually so shaped as to have a long concave surface with curvature in the longitudinal direction such that ultrasonic energy can be effectively focused. In other words, the diameter L of the lens must be quite large. The acoustic lens is usually made of aluminum, acryl or polystyrene. Since the speed of sound in such materials is greater than inside water, ultrasonic waves can be focused by a lens with such a concave surface.
The method of focusing ultrasonic energy shown in FIG. 3 is disadvantageous in that a very complicated electronic circuit is required for the necessary phase control and this affects the cost of the apparatus. Since as many lead lines are required as there are transducers in the array, furthermore, the diameter of the applicator also increases accordingly and the possibility of wire breakage also increases. The focusing method by an acoustic lens as shown in FIG. 4 is advantageous because the electronic circuit can be simplified and the number of components to be assembled is reduced, thereby also lowering the cost of manufacture. On the other hand, however, this method imposes a severe restriction on the shape when it is applied to an ultrasonic applicator for use inside a body cavity. For example, an applicator for use inside body cavities must be about 20-25mm in diameter. Let us assume that the diameter L of the acoustic lens of Fig, 4 is 60mm and that the lens is made of aluminum, the speed of sound therethrough being 6400m/sec. The radius of curvature r of the acoustic lens and the distance F of its focus therefrom are related as follows: r=F(1-c.sub.2 /c.sub.1), where c.sub.1 is the speed of sound inside the acoustic lens and c.sub.2 is the speed of sound inside the medium into which the sound is emitted from the lens. In a situation where this applicator is thrust through the anus to treat the hypertrophy of the prostate gland (or benign prostatic hyperplasia "BPH") by transrectally heating, for example, ultrasonic energy must be focused about 40mm in front of the ultra sound-radiating surface of the acoustic lens, and r becomes approximately 30.6 mm according to the formula given above if the medium is assumed to be water such that the speed of sound therein is 1500 m/sec. This means that the thickness (shown by letter t in FIG. 4) of the lens at its edges becomes very large. If an elongated cylindrical ultrasonic wave applicator for use inside body cavities is designed with a lens of this size, the applicator will be too thick to be inserted into a human body cavity such as the rectum. In other words, there is a severe dimensional limitation on transducers and acoustic lenses having a curvature (or focal length) suitable for an ultrasonic wave applicator for use inside body cavities.
One may think of reducing the diameter L of the acoustic lens such that the thickness t can be reduced, but this will also reduce the surface area of the transducer. Since there is a limit to the power output per area of the transducer, the total area of the energy-emitting surface cannot be reduced too much because it is necessary to obtain sufficient ultrasonic energy for a medical treatment must. In other words, there is a limit to how small the diameter L can be made.
It is also to be noted that, since-the target region for medical treatment usually extends in the radial direction of the emitted ultrasonic waves, it will be advantageous if the ultrasonic waves can be focused simultaneously at more than one target position because the effective region of treatment is thereby increased and the effect of treatment improves. Moreover, the time required for treatment can be shortened and, since the applicator does not have to be moved inside the body cavity in order to change its focal point, the pain to the patient can be alleviated.